Motional quantum state engineering for quantum logic spectroscopy of molecular ions

authored by: Fabian Wolf
supervised by: Piet O Schmidt
Abstract: High precision spectroscopy of molecular ions is a promising tool for the investigation of fundamental physics. Potential applications are the search for a possible variation of fundamental constants, the measurement of an electron electric dipole moment or the direct measurement of a frequency shift caused by parity violation in chiral molecules. The complex structure of molecules including the additional degrees of freedom, namely rotation and vibration, in general results in a dense level structure and the absence of closed cycling transitions. Consequently, techniques that have enabled high precision laser spectroscopy on atomic ions, such as direct laser cooling, optical pumping and fluorescence detection, are not applicable to most molecular ions. This obstacle can be overcome by employing quantum logic techniques to enable precision spectroscopy on molecules. For this purpose, the molecular ion is co-trapped with an atomic ion that has a suitable transition for laser cooling and state detection. The Coulomb repulsion couples the motional degrees of freedom of the ions. This coupling enables sympathetic cooling and information transfer from the molecule's internal state to the atomic qubit. In this cumulative thesis, the first experimental implementation of a quantum logic assisted scheme for reading out the internal state of a molecular ion is presented. In this scheme, the magnesium-25 ion is used to detect a state dependent force that acts on the molecular 24MgH+-ion. Furthermore, a quantum-enhanced force sensing protocol is demonstrated, which can be applied to the previously described molecule measurement, but has further applications in the more general field of quantum metrology. A salient feature of the presented quantum sensing protocol is that the metrological gain, achieved by employing the quantum features of the ion's motional state, is independent of the phase of the measured oscillating force. This is a substantial difference to previously demonstrated schemes based on squeezed states or Schrödinger cat states, and allows probing of two conjugate variables with sensitivities below the classical limit using the same initial quantum state.
Organisation(s): QUEST-Leibniz Research School
External Organisation(s): National Metrology Institute of Germany (PTB)
Type: Doctoral thesis
No. of pages: 102
Publication date: 2019
Publication status: Published
: Details in the research portal "Research@Leibniz University"

BibTeX

@phdthesis{0af44410441141fea7e42b48bca9a29e,
title = "Motional quantum state engineering for quantum logic spectroscopy of molecular ions",
abstract = "High precision spectroscopy of molecular ions is a promising tool for the investigation of fundamental physics. Potential applications are the search for a possible variation of fundamental constants, the measurement of an electron electric dipole moment or the direct measurement of a frequency shift caused by parity violation in chiral molecules. The complex structure of molecules including the additional degrees of freedom, namely rotation and vibration, in general results in a dense level structure and the absence of closed cycling transitions. Consequently, techniques that have enabled high precision laser spectroscopy on atomic ions, such as direct laser cooling, optical pumping and fluorescence detection, are not applicable to most molecular ions. This obstacle can be overcome by employing quantum logic techniques to enable precision spectroscopy on molecules. For this purpose, the molecular ion is co-trapped with an atomic ion that has a suitable transition for laser cooling and state detection. The Coulomb repulsion couples the motional degrees of freedom of the ions. This coupling enables sympathetic cooling and information transfer from the molecule's internal state to the atomic qubit. In this cumulative thesis, the first experimental implementation of a quantum logic assisted scheme for reading out the internal state of a molecular ion is presented. In this scheme, the magnesium-25 ion is used to detect a state dependent force that acts on the molecular 24MgH+-ion. Furthermore, a quantum-enhanced force sensing protocol is demonstrated, which can be applied to the previously described molecule measurement, but has further applications in the more general field of quantum metrology. A salient feature of the presented quantum sensing protocol is that the metrological gain, achieved by employing the quantum features of the ion's motional state, is independent of the phase of the measured oscillating force. This is a substantial difference to previously demonstrated schemes based on squeezed states or Schr{\"o}dinger cat states, and allows probing of two conjugate variables with sensitivities below the classical limit using the same initial quantum state.",
author = "Fabian Wolf",
note = "Doctoral thesis",
year = "2019",
language = "English",
series = "PTB-Bericht",
publisher = "Leibniz Universit{\"a}t Hannover",
school = "Leibniz University Hannover",
}